1. Field of the Invention
The present invention relates to short term residual dust suppression, and more particularly to such suppression by use of an aqueous mixture of a foaming agent and a wetting agent.
2. Description of the Prior Art
Dust formation from a variety of sources has been a continuing cause of environmental and health concerns. Particular attention has been paid to the dust developed from the handling of coal, but such sources also include, for example, petroleum coke, recycled glass dust and bauxite. Thus, while in this specification, reference is often made to coal, it should be understood that this discussion is applicable to numerous other dust sources as well.
The various industries affected by such dust formation have engaged in many efforts to avoid or to alleviate the problem of dust formation that results during handling, conveyance, transportation and even storage of coal and the other dust sources. Typically, such efforts involve the use of water incorporated into a chemical dust suppressant applied to the coal or other source. Conventionally, although the categories of dust suppressants have overlapped to some extent in that certain types of suppressants may be reformulated to be applied through a system designed for another type, a suppressant may fall into the category of a short term dust suppressant, which may be a wet suppressant or a foam suppressant, or a long term residual dust control suppressant. Short term dust suppressants function by coating the source and dust with water. Thus, such suppressants lose their effectiveness upon evaporation of the water. Moreover, their effectiveness generally does not persist beyond one to two impact points; that is, points at which dust is generated during handling or movement of the coal or other source. Examples of wet and foam suppressants are discussed in U.S. Pat. Nos. 4,737,305 to Dohner, 4,836,945 to Kestner and 4,971,720 to Roe, and in Surfactants and Interfacial Phenomena, 2d Edition, by Milton J. Rosen, Wiley Interscience Publications (1989), all of which are incorporated herein by reference. Long term residual dust suppressants control dust by means of the formation of a polymer or binder film over the dust source and thereby persist even after evaporation of water in the suppressant. The present invention relates to a new type of short term dust suppressants that may be applied as a wet suppressant or as a foam suppressant.
Wet suppression is based on what is called "wet technology". This suppression can be as simple as spraying large amounts of water on the coal (or other source) as it travels along a conveyor or drops to a storage pile or transfer bin. However, although water is an effective dust suppressant, its use involves a number of drawbacks, such as adding weight to the coal or other source (which can result in higher costs for transporting the source), development of substantial vapor pressure when the coal or other source is heated or burned, and the absorption and thus wasting of substantial energy as the water vaporizes when the coal or other source is heated or burned.
As a result, surfactants or "wetters" often are added to the water to reduce the amount of the water needed for dust suppression. Conventional wetters include nonionic epoxide (e.g., ethylene oxide or propylene oxide) derived block co-polymers, alcohols of from about eight to about sixteen carbons ethoxylated with from about four to about ten moles of ethylene oxide (wherein the alcohol may be an aromatic such as alkyl phenol, preferably nonylphenol, which can be ethoxylated with, for example, ten moles ethylene oxide), and branched nonionic surfactants such as branched alpha sulfo ester salts comprising an acid chain and an alcohol chain of from about six to about twelve carbons each, and highly branched alcohol sulfate and alcohol ether sulfate detergents. Generally, the wetter is added to the water in a weight proportion of from about 0.2 to about 5 parts of the wetter to about one thousand parts of water. As this concentration, the wetter acts at the interface between the coal (or other source) and the water to increase the affinity of the coal and water, thereby decreasing the amount of water needed to soak the coal as well as decreasing the time required for the water to penetrate the coal stream.
Typically, such wet technology is employed to suppress dust Generated at transfer points, areas where the coal falls freely from one point to another (free falls) such as loading points where the coal is dropped into a vessel for transportation, impact points where the coal strikes a surface, and storage areas. The water is applied at the point of dust generation and is applied to the air-borne dust as well as to the source of the dust. If the amount of water added to the coal is sufficiently great, the coal can be prevented from dusting significantly on impact. For such benefits, the water should be added in an amount sufficient to result in a proportion of one to three parts by weight water per one hundred parts by weight wetted coal. However, because the suppressant is effective only through one or two impact points where dust could be generated, repeated applications are necessary, thereby increasing the water content of the wetted coal quickly to seven or eight percent.
Foam suppressants are applied to form a blanket over the coal or other source to capture and smother dust. Bubbles in the foam suppressant catch the dust particles and so the foam suppressant is effective only until the bubbles break or the layer of foam becomes discontinuous. The foam suppressant is formed by addition of a roamer to water. Conventional foamers include alpha-olefin sulfonates, alkylphenyl sulfonates with long alkyl chains (e.g., eight to sixteen carbons) such as sodium lauryl benzene sulfonate, alcohol sulfates, alcohol ether sulfates, alpha sulfo esters and mixtures of such compounds. Under standard conditions, from about one to about twenty parts by weight foamer is added to about one thousand parts by weight water. The resulting foam has about five to ten percent of the density of the water used in wet technology and so much less water is needed for a foam to provide the same volume of applied dust suppressant as the wet suppressant. Thus, the foam suppressant can be added to the coal in a proportion such that the wetted coal contains 0.2 to 0.5 parts by weight added water per one hundred parts by weight wetted coal. However, as with the wet suppressant, the foam suppressant is effective only through one or two impact points where dust could be generated. Thus, repeated applications are necessary and the water content of the wetted coal increases quickly to several percent.
As with wet suppressants, the foam suppressants are employed to suppress dust Generated at impact or transfer points, areas where the coal falls freely from one point to another (free falls) such as loading points where the coal is dropped into a vessel for transportation, impact points where the coal strikes a surface, and storage areas. The foam is applied as a curtain or barrier to capture Generated dust. The foam applicator nozzles are located in such a way that the remaining foam and the captured dust are deposited back onto the moving coal stream. This orientation not only prevents dust from escaping into the environment, but also places at least a partial blanket of foam onto the deposited coal, which may prevent dust Generation until bubbles are broken or disrupted by another transfer point. The dust suppression effects of normal foam does not carry over from a previous application point to further impact zone.
Long term residual dust control suppressants are used to prevent Generation of dust during long term storage or during transportation. Such long term residual suppressants operate by a mechanism very different from those of short term residual suppressants to which the subject invention is directed. In short term residual (wet or foam) suppression, the water eventually evaporates, rendering the suppressant ineffective in suppressing dust over a longterm, such as during several months of outdoor storage. Thus, long term residual dust suppressants remain active long after the water evaporates. They ordinarily have some film-forming or tackifying properties. For example, U.S. Pat. No. 4,801,635 to Zinkan et al. describes a long term residual dust suppressant that includes an acrylic polymer and U.S. Pat. No. 4,169,170 to Doeken describes a long term residual dust suppressant that includes an aqueous suspension of asphalt emulsion concentrates or black liquor lignin products as a binder material. Conventionally, water is included in a long term residual suppressant typically not only to provide some dust control during handling prior to storage, but also to promote even spreading as the suppressant is applied. Long term residual dust suppressants often contain wetters or foamers as well to minimize the amount of water needed to apply the suppressant to the coal and are applied directly to the coal in what is known as a "main body treatment".
Thus, several problems are associated with conventional wet and foam dust suppression techniques to which the subject invention is directed. For example, each technique involves addition of a substantial amount of water to the coal or other dust source, especially in view of the repeated applications of water-based suppressant necessary to control dust across several impact or transfer points. The resulting high water content of the coal is particularly undesirable in that much dust suppression is performed at fossil fuel power plants. Water added to the fuel results in a portion of the heat output of the coal to be lost to vaporization of the water and so a loss of effective energy. The vaporization of water consumes substantial amounts of heat. Therefore, the addition of such significant amounts of water is particularly troublesome. In addition, the additional water increases the weight of the coal and so increases shipping costs accordingly.
Because of the substantial disadvantages associated with the addition of such significant amounts of water, the industry has attempted to minimize the amount of water employed in wet and foam suppression techniques. Such attempts typically involve the use of systems for application of the suppressant at each dust producing site instead of a single application that would be intended to last through many transfer points during transport or conveyance of the coal or other dust source. Because the suppressant in the multiple application technique remains effective for only one or two transfer points, such techniques are expensive; they require costly installation of application equipment at several transfer points, impact points and loading or "stack-out" storage sites. In addition, impact sites where dust is generated often are not accessible to the equipment employed in conventional application systems. Thus, such techniques are undesirable.